1. Field of the Invention
The present invention relates to an optical fiber suitable for transmitting signal-carrying light waves in a 1.55-μm-wavelength band, an optical transmission line incorporating the optical fiber, and an optical communications system incorporating the optical fiber.
2. Description of the Background Art
An optical communications system can transmit and receive a large volume of information by transmitting signal-carrying light waves over an optical fiber. Furthermore, the volume of information can be increased by using an optical communications system employing a wavelength division multiplexing (WDM) transmission system, which transmits a plurality of optical signals having different wavelengths by multiplexing them. However, ever-increasing communication demands in recent years have been requiring a further increase in the volume of information to be transmitted. In order to meet this requirement, researchers and engineers have been conducting studies to decrease the separation between the neighboring wavelengths in WDM signal-carrying light waves, to broaden the wavelength band of the light waves, and to increase the bit rate of each optical signal.
When the volume of information is increased, it becomes more important to control the chromatic dispersion of optical transmission lines. For example, it is desirable that the absolute value of the cumulative dispersion of an optical transmission line be small in the wavelength band of the signal-carrying light waves, because the wave form of the signal-carrying light waves propagating the optical transmission line degrades due to the interaction between the cumulative dispersion of the optical transmission line and the self phase modulation, a type of nonlinear optical phenomena. On the other hand, a small absolute value of the chromatic dispersion increases noises due to four-wave mixing, another type of nonlinear optical phenomena. Therefore, it is desirable to avoid an excessively small absolute value in the chromatic dispersion of an optical transmission line at the wavelength band of the signal-carrying light waves. Consequently, studies taking the chromatic dispersion into account have been conducted on optical transmission lines capable of transmitting signal-carrying light waves with high quality, and some of the study results have been reported.
A typical single-mode optical fiber has a core made of silica glass doped with germanium oxide (GeO2). The optical fiber has a chromatic dispersion of about 17 ps/nm/km at a wavelength of 1,550 nm, whereas some optical fibers having a smaller chromatic dispersion at the same wavelength have been reported.
For example, U.S. Pat. No. 5,835,655 has disclosed an optical fiber having a zero dispersion wavelength of 1,531 nm and a small chromatic dispersion at a wavelength of 1,550 nm. Another example is given in U.S. Pat. No. 5,327,516, which has disclosed an optical fiber having a chromatic dispersion of about 2 ps/nm/km at a wavelength of 1,550 nm. K Mukasa et al. have disclosed an optical fiber having a chromatic dispersion of 14.7 ps/nm/km at a wavelength of 1,550 nm in a paper entitled “Wide-Band Dispersion Management Transmission Line with Medial Dispersion Fiber (MDF)” (ECOC'2000, pp. 95 –96). K Aikawa et al. have disclosed an optical fiber having a chromatic dispersion of 9.2 ps/nm/km at a wavelength of 1,550 nm in a paper entitled “New Dispersion-Flattened Hybrid Optical Fiber Link Composed of Medium-Dispersion Large-Effective-Area Fiber and Negative Dispersion Fiber” (OFC'2001, TuH6).